The use of lenses for correcting vision problems produced by deficiencies in the optical system of the human eye has been known for many years. FIG. 1A illustrates, in schematic form, the anatomy of the human eye 10. Light enters eye 10 through cornea 12, passes through lens 14, and strikes retina 16, the light-detecting inner surface of the eye. The fovea 18 is a central region of retina 16 having particularly high acuity. Lens 14 is attached around its periphery to zonular fibers 20. Zonular fibers 20 are connected to ciliary body 22. Ciliary body 22 is a sphincter muscle which opens when it is relaxed, thereby generating tension in zonular fibers 20. Ciliary body 22 releases tension on zonular fibers 20 when it is contracted. Lens 14, because of its inherent elastic properties, tends to assume a rounded form when it is not subject to external forces. Thus, when ciliary body 22 contracts, lens 14 becomes more rounded, while relaxation of ciliary body 22 produces flattening of lens 14. Cornea 12 provides a significant portion of the refractive power of the optical train of the eye, but the capacity for accommodation is contributed by lens 14.
FIG. 1B illustrates a relaxed (unaccommodated) eye 10, in which lens 14 is flattened. As indicated by the solid lines in FIG. 1B, light from distant objects will be focused on retina 16 (and specifically, on fovea 18) by lens 14, but light from near objects (indicated by the dashed lines) will be focused behind the retina, and thus appear out of focus at the retina. FIG. 1C illustrates an accommodated eye 10, in which lens 14 has assumed a more rounded form. In the accommodated eye, light from near objects (indicated by dashed lines) is focused on retina 16 (fovea 18), while light from distant objects (indicated by solid lines) is focused in front of the retina, and thus is out of focus at retina 16.
In a normal, healthy eye, adjustment of lens 14 is sufficient to focus images on retina 16 within a wide range of distances between the visual target-object and the eye. Myopia (near-sightedness) and hypermetropia (far-sightedness) occur when images entering the eye are brought into focus in front or in back of the retina, respectively, rather than exactly on the retina. This is typically caused by the eyeball being too long or too short relative to the focal-adjustment range of the lens. Eyeglasses with spherical focusing lenses of the appropriate optical refractive power can be used to compensate for myopia or hypermetropia.
Another common and readily corrected visual problem is astigmatism, a focusing defect having orientation-dependence about the optical axis of the eye that may be corrected by interposition of a cylindrical lens having appropriate refractive power and axis-angle of orientation. Other visual focus problems exist as well (e.g., coma and other higher order optical aberrations), but are less readily characterized and more difficult to correct in a practical manner. In general, focal problems caused by irregularities in the dimensions of the cornea, lens, or eyeball can be corrected, providing the optical properties of the eye can be characterized and a suitable (set of) optical element(s) manufactured and then positioned relative to the eye.
Aging subjects may experience presbyopia, a decrease in the ability to focus on proximate visual targets caused by reduced flexibility of the eye lens relative to the tractive capabilities of the operative musculature attached thereto. Difficulty in focusing on such proximate visual targets can be alleviated with the use of ‘reading glasses’. Subjects who require correction for myopia as well as presbyopia may use “bifocal” glasses having lens regions that provide correction for both ‘near’ and ‘far’ vision. The subject selects the type of correction by looking toward the visual target through the appropriate portion of the lens. Elaborations and extensions on such systems are now common, including “trifocal glasses” and “progressive glasses,” the latter featuring a continuous gradation in optical properties across a portion of the eyeglass and thus of the visual field thereby regarded.
Adjustable optical systems are used in a wide variety of devices or instruments, including devices that enhance human vision beyond the physiological range, such as telescopes, binoculars, and microscopes, as well as a numerous devices for scientific and industrial applications independent of human vision, such as in test, measurement, control, and data transmission. Such devices typically make use of complex systems of multiple lenses and optical components that are moved with respect to each other to provide a desired level of focus and magnification. Adjustable lens systems that have been proposed for use in eyeglass-type vision enhancement include electroactive lenses, as described in U.S. Pat. Nos. 6,491,394 and 6,733,130 and various types of fluid lenses, as described in U.S. Pat. Nos. 4,466,706 and 6,542,309, as well as assorted multi lens systems (see e.g., U.S. Pat. Nos. 4,403,840 and 4,429,959).
Various methods have been developed for measuring neural activity. Electrical measures of neural activity can be obtained from electrodes positioned within a neural structure to record activity from one or a few cells, or from electrodes placed on a skin surface to measure electrical fields, typically representing the activity of multiple cells. Magnetic fields generated by the nervous system can also be measured by devices such as SQUIDs (superconducting quantum interference devices), which may be placed on the scalp to measure magnetic fields from the brain. Other methods, such as magnetic resonance imaging and optical (spectroscopic) measurements permit neural activity to be determined indirectly by measuring correlates of brain activity such as blood flow or metabolic activity. Similarly, muscle activity can be assessed through measurement of electrical and magnetic fields generated by muscles (e.g., electromyographic measures or EMG), as well as by measurements of muscle force, displacement, or related parameters.